Conventionally, honeycomb structures have been used, for example, in an exhaust gas purifier of a heat engine (e.g. an internal combustion engine) or a burner (e.g. a boiler), or in a reformer of a liquid fuel or a gaseous fuel. It is known that honeycomb structures are also used to capture and remove a particulate substance present in a particle-containing fluid, for example, an exhaust gas emitted from a diesel engine.
In the honeycomb structure used for such a purpose, the sharp temperature change of exhaust gas and the local heating tend to make non-uniform the temperature distribution inside the honeycomb structure, which has caused problems such as crack generation in honeycomb structure and the like. When the honeycomb structure is used particularly as a filter for capturing a particulate substance in an exhaust gas emitted from a diesel engine, it is necessary to burn the fine carbon particles deposited on the filter to remove the particles and regenerate the filter and, in that case, high temperatures are inevitably generated locally in the filter; as a result, a big thermal stress and cracks have tended to generate.
Honeycomb structures have become larger depending upon the application purpose. Hence, it is known to produce a honeycomb structure by bonding a plurality of honeycomb segments. In this case as well, it is necessary to reduce the thermal stress generated.
As a means for reducing the thermal stress, there is disclosed in, for example, U.S. Pat. No. 4,335,783, a process for producing a honeycomb structure, which comprises bonding a large number of honeycomb parts using a discontinuous adhesive. Also in JP-B-61-51240 is proposed a heat shock-resistant rotary regenerating heat exchanging method which comprises forming, by extrusion, matrix segments of honeycomb structure made of a ceramic material, firing them, making smooth, by processing, the outer peripheral portions of the fired segments, coating the to-be-bonded areas of the resulting segments with a ceramic adhesive having, when fired, substantially the same mineral composition as the matrix segments and showing a difference in thermal expansion coefficient, of 0.1% or less at 800° C., and firing the coated segments. Also in a SAE article 860008 of 1986 is disclosed a ceramic honeycomb filter obtained by bonding cordierite honeycomb segments with a cordierite cement and, in this literature, there is disclosed a bonding method for obtaining discontinuous bonded areas. Further in JP-A-8-28246 is disclosed a ceramic honeycomb filter obtained by bonding honeycomb ceramic members with an elastic sealant made of at least a three-dimensionally intertwined inorganic fiber, an inorganic binder, an organic binder and inorganic particles.
Meanwhile, the regulation for exhaust gas has become stricter and engines have come to have higher performances. As a result, in order to achieve an improvement in combustion conditions of engine and an increase in purification ability of catalyst, the temperature of exhaust gas has increased year by year. In this connection, a higher thermal shock resistance has come to be required for honeycomb carriers. Therefore, even with honeycomb structures such as mentioned above, when there is a bigger heat generation during their regeneration, there may arise problems such as generation of cracks, etc. in the applied adhesive or the bonded areas.
Honeycomb structures can have a higher strength by making their partition walls thicker; however, it results in a larger pressure loss and impairment of engine performance, etc. Hence, in JP-B-54-110189 is proposed a honeycomb structure in which the partition wall thickness of honeycomb carrier is made smaller regularly in the cross-section toward the sectional center; and in JP-A-54-150406 and JP-A-55-147154 is proposed a honeycomb structure in which the partition wall thickness of the cells in the outer peripheral portion of the honeycomb structure is made larger than the partition wall thickness of inner cells. These honeycomb structures have a large strength to the stress applied from outside; however, they have no sufficient durability to such a thermal stress as appears when the center of honeycomb structure, in particular, comes to have high temperatures during the use. Further, in these literatures, no mention is made on the partition wall or side wall of each honeycomb segment of a honeycomb structure obtained by bonding of a plurality of honeycomb segments, particularly each honeycomb segment located in the inner portion of the honeycomb structure.
The aim of the present invention is to provide a honeycomb structure wherein the temperature rise during the use is suppressed and the durability to the cracks caused by thermal stress appearing therein during the use is improved further while an increase in pressure loss and a reduction in functions (e.g. purification ability) are suppressed.